Internally conductively coated lamp and method of manufacture



Aprll 1, 1958 M. GAST ETAL 2,829,295

INTERNALLY CONDUCTIVELY COATED LAMP AND METHOD OF MANUFACTURE Filed July 12, 1950 Inventors: Theodoms AM Gas tr, Frederik Hans Jan van Hoom,

We KW Their A 'lr korneg.

United rates Patent 2,s29,z9"'s;' I in AND METHOD OF MANUFACTURE Theodorus' Adrianiis Maria has: and} Frederik Jan van Hoorn, Eindhoven,'Netherlands,' assignors," by mesne assignments, to North. American" Philips. Company, Inc., New York, N. Y., a corporatidnof Delaware i 1 1 Application 12} l 9 fiiog sei'i affidi Claims priority, application Netherlands July 2, 1949* 1 p 6 ciatias: diets-'16s) The present inventienrelatesto gaseous e'lectric'de vices generally; and more pa'rticularlyhto an improved fiuorescent'lamp construction wherein the inter-nah surface of-thelamp is provided with a conductive c'oating or"strip which operates as a starting aid: The-invention is'also concerned with a method of applying the coating for securing good adhesion to ats'urface and a; desired low valueofresistancel i i i It is relatively well known that the'striking orstart ing of electrical'dis'charge devices;-such as gas'or vapor tubes,- may be facilitated by providing anelectrically co n ductive coating on eithe'r'the'inner ,or' theouter' surface 2,829,295 atria Patented sintering, the adherence is improved.

Thus, by utilizing graphite mixed in with glazing powder and sintering', good adherence may be obtained but' other disadvantages ensue; These disadvantages;-' which have'been discoveredduring thecourse of certain experiments from which the presentinvention' has-ire: sulted, stem from an apparently unavoidable increase in ofthe' envelope of the device. The conductive coating may remain unconnected" and" operate solely byfcapaci tive effect, or it may be connected-to oneor-more' of the electrodes in the device; In the case'of elongated tubular fluorescent lamps'for illuminating purposes; the conduc-j tivecoating may be formed as" a narrow strip" or band along thelength of thelampyor it may beatransparent coating: covering the suffacebf the lamp either externally oririternally.

When it is desired to apply a'n'internal conductivecoat luminescent layer with-the coriductive coating in suehj manner as to retain the eifeetivenessof'both'layers arises." It-is possible toappl'y the. conductive layer first" and then to apply the luminescent layerover it. :Wit hsficha'con struction, howeven'thepresence of the luminescent layer reduces the effectiveness of the conductive layer and" other hand; if the 'luriiinesdentlayeris appliedfirsfand" sorbed locally. This" conductive strip may be 'sin'tered i be' loweredvery marke'dly'whenafiln'iior skiii p'roducifig" on the" interior surface of-the discharge tube or may applied thereto in a molten condition: It hasbeen ip: posed furtherwto use glazing-powder as the inbrganic bondingmeans; zinda'ptiwderd metal, for ert'ample' silver, asthe conductive substance in thedisp ersion.'

Theme of a; metal in such a binder has the vantage" that-stresses are produced 1 in the envelope 1 wells" asareshlt of the'application of'heat necessary topi'odiic the? bonding: Moreover; in" discharge tubes wherein mercury is pfesefit which, in practice, comprises 'a great majority of commercially produced discharge lamps; the

mercury? is absorbed by theic'c'mcluctiv'e coating. Another disadvantage is the necessity ofra'ising the'tempe'ra tu'r'ej of the" tube during the manufacturing? process to a' point which is-quite critical-and which must be" accurately" maintained. l H I The limitations or disadvantages" enumerated: above,

instead ohmetal. Howevetgait is -nbta"sinip1e'mhtti total resistance of a conductive coating, from end to end of a commercial low pressure discharge device, such as" a-48-inch, 40 watt fluorescent lamp, in" the'rang'e'betv veen 1000 and 3000'ohrns. It might also bementioned that simply reducing the quantity of the glazingpowde'r and; proportionally increasing the quantity'of' graphite does not solve theproblem, because the bonding then be comes'insufficient. h h

Accordingly, it is anobject of our inventionftopio vide anew and improved internal conductive coating or strip'fo'r an electric discharge device'whicli has good" adherence and does not impmr the performance or the life of the: device.

Another object of our invention is to provide'a-"di's charge lamp with an internal conductive strip of low resistance, good adherence, and producing only negligible absorption of mercury vapor.

surface of. adisch'arge lamp.

The invention is based on our discovery of thu'nii pected" phenomenon that the resistance of a conductive V coating which is made from a suspension of graph an'organic liquid, for example, butyl'oi' amyl acetate, ay

substance is dissolved previously in the organic, liquid Suchafilrmproducing substance may be, for instance cellulosic ether or ester, such as cellulose nitrate, also known as nitro-cellulose. "thus," when the"s"u spefi'sion of graphite ismade in a solution of a filni-prciduiziiig substance in an organic liquid, such being 'cellu in butyl-acetate for the examples givenab'ove, th re sistance' of the conductive coating produced of the process mentioned herein is, miic'hld' resistance of an identical conductive coating prepared n the same way as a suspension but without'the T producing substance. This makes it pds sible to add sufiicient quantity of glazing 'poyvderwhich; thifg it does'raise thelresi st'anc e of'th coatingi'does not now: need to raise it above the optimum value'r ciuired 'fdr meta-venues. V

stem presses 3 and 4, stem 3 being further provided with an exhaust tube 3' for evacuating the envelope and introducing suitable gases therein. Mounted on lead wires passing through the stem presses are filamentary electrodes 5 and 6, which may consist of a coil of tungsten wire overlaid with a coating of activated electron-emitting materials such as barium and strontium oxides. The electrode lead-in wires are connected to a pair of pins. 7 and 8 fixed in suitable molded bases at both ends of the lamp. The interior surface of the envelope is coated with. athin layer 2 of a fluorescent substance, whose function is to convert the ultraviolet radiation produced by the discharge to light radiation within the visible spectrum. In an actual construction, the tube 2 may have a length of approximately 126 centimeters and a diameter of 36 millimeters, which size wouldcorrespond in commercial practice to a 40-watt fluorescent lamp.

Disposed longitudinally along the inside surface of the tube 2 is a thin conductive strip 9 which may be approximately 2 millimeters wide. The strip runs the length of the lamp to points in the vicinity of the electrodes 5 and 6. ,The strip consists of a mixture of graphite in a low melting point glazing compound, the graphite being originally provided asa suspension in a film-forming solution in an organic solvent mixedwith the glazing compound. This is in order to obtain a low resistance value for the conductive strip after sintering. The resistance of the strip from end to end maybe in the range between 1000 and 3000 ohms. The conductive strip may be unconnected to any other part of the lamp; or it may, if desired, be connected to one of the electrodes as by a spring finger 10 mounted on one of the lead-in wires of electrode 5.

In its final assembly, the lamp is evacuated, filled with a starting gas such as neon, argon, krypton, xenon, or mixtures thereof at a low pressure in the neighborhood of 3 mm. In addition, a droplet of mercury 11 is introduced into the bulb, which, during normal operation, fills the envelope with its vapor pressure at the operating temperature, such vapor pressure being in the range of a few microns.

The coating mix which eventually constitutes the conductive strip 9 may be applied to the inside surface of the tube 2 by utilizing a small wheel whose running surface is covered with a supply of the coating mix. The small wheel may be caused to roll over the surface of the luminescent layer thereby to apply the thin narrow strip.

The coating mix may be prepared in the following manner: 200 grams of graphite are ground over a period of 96 hours in a porcelain ball mill together with 1100 cubic centimeters of a 1% solution of cellulose nitrate in butyl-acetate. Subsequent to this operation, 500 grams of low melting point glazing powder are added, and further grinding of the mixture is provided for another 24 hours. The glazing powder may consist of 80.8% PbO, 16.4% B 0 and 2.8% SiO The suspension thus obtained is then removed from the ball mill, and the interior. of the mill and also the balls are washed with 40 cc. of butyl acetate as a rinsing. liquid, which rinsing liquid is afterwards added to the main portion of the suspension and stirred in. The viscosity of the suspension measured at this point by means of the Ford cup method with an orifice of 5.7 mm. gives a reading of 18 to 20 seconds.

After a strip of the coating mix or suspension obtained in the manner described has been applied along the interior surface of the tube, as by rolling small wheel coated with the mix once over the luminescent layer, the tube is placed in an oven at a temperature slightly in excess of the softening point of the glass and high enough to cause sintering of the coating, for instance, at approximately 500 C. for a period of three minutes. After the tube has been removed from the oven and allowed to cool, it may be utilized in known manner for the manufacture of a fluorescent lamp, such as is illusrated in the drawing.

The resistance of an actual construction of a conductive strip 116 cm. long and 2 mm. wide, made in the manner described above and measured after cooling, is about 2000 ohms, and the resistance does not change materially during the succeeding operations for manufacturing the fluorescent lamp. Taking into account the previou'sly given dimensions, the resistance of the strip is approximately 33 ohms per centimeter length of strip 1 millimeter wide. vI

The construction and method of manufacture which have been described provide considerable advantages over the constructions and processes utilized heretofore. For one,.it.permits the .provision of a low resistance. strip within the optimum range required for maximum lowering of the starting voltage of the lamp, and which, nevertheless, adheres well to the envelope walls. A strip containing a graphite suspension as described, does not react with the mercury within .the envelope so that the performance of the lamp during its life is not impaired. Moreover, a coating mix or suspension prepared in accordance-with the method described fiowseasily, so that .it may be applied to produce an even covering for the strip". Finally, the use of graphite rather than metal as the conductive material within the strip eliminates the destructive stresses which would otherwise be produced in the glass walls of the lamp.

1 While a certain specific construction and a specificprocess of manufacture have been shown and described, it will, of course, be understood that various modifications may be made without departing from the invention. Thus, the example of a film-forming substance is merely illustrative and other known substances may be substitutedtherefor, as likewise for the solvent and the glazing powder. vThe appended claims are, therefore, intended to cover any such modification coming within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

- l. A method of providing a starting strip for an electric gaseous discharge tube including a glass envelope,- a' pair of spaced electrodes in the envelope, and a lumi-- nescent layer on the internal surface of the envelope which comprises forming a suspension of finely-ground graphite in an organic solvent containing an organic heatdecomposable film-forming material, mixing said suspension with a glazing powder, applying a thin layer of said mixture over a selected portion of the surface of the luminescent layer between the electrodes, and sintering said layer on said luminescent layer at an elevated temperature to produce a firmly-adherent conductive strip on said luminescent layer.

2. A method of providing a starting strip for an electric gaseous discharge tube including a glass envelope, a pair of spaced electrodes in the envelope, and a luminescent layer on the internal surface of the envelope which comprises forming a suspension of finely-ground graphite in an organic solvent containing a heat-decomposable film-forming cellulose ester, mixing said suspension with a glazing powder, applying a thin layer of said mixture over a selected portion of the surface of the luminescent layer between the electrodes, and sintering said layer on said luminescent layer at a temperature slightly in excess of the softening point of the glass to produce a firmly-adherent conductive strip on said luminescent layer.

3. A method of providing a starting strip for an electric gaseous discharge tube including a glass envelope, a pair of spaced electrodes in the envelope, and a luminescent; layeron the internalsurface of. .the. envelope which comprises forming a suspension of finely-ground graphite in an organic solvent containing an organic heat-decomposable film-forming material, mixing said suspension witha glazing powder in proportions at which the resulting mixture contains at least 1 part of graphite to 2 parts of glazing powder, applying a thin layer of said mixture over a selected portion of the surface of the luminescent layer between the electrodes, and sintering said layer on said luminescent layer at a temperature slightly in excess of the softening point of the glass to produce a firmly-adherent-conductive stripon said luminescent layer.

4. A method of providing a starting strip for an electric gaseous discharge tubeincl-uding an envelope, a pair oftspaced electrodes in the envelope, and a luminescent layer on the internal surface of the envelope which COB]- prisesforming a suspension of finely-ground graphite in an organic solvent containing an organic heat-decomposable film-forming material selected from the group consisting of cellulose esters and cellulosetethers, mixing said suspension with a glazing powder in proportions at which the resulting mixture contains 1- part of graphite to 2 parts of glazing powder, applying a thin layer of said mixture over a selected portion of the surface of ing as a film-producing agent, mixing said suspension with a glazing powder comprising lead oxide, boric oxide and silicon dioxide, the proportions of graphite and glazing powder in the mixture being in excess of 1 to 2, applying a thin layer of said mixture over a selected portion of the surface of the luminescent layer between the electrodes, and sintering said layer on said luminescent layer at a temperature of approximately 500 C. to produce a firmly adherent conductive strip on said luminescent layer.

6. A method of providing a starting strip for an electric gaseous discharge tube comprising an envelope, 3. pair of spaced electrodes in the envelope and a luminescent layer on the internal surface of the envelope which comprises grinding graphite together with a solution of approximately 1% cellulose nitrate in an organic solvent to form a fine suspension, further grinding said suspension with a quantity of a low-melting point glazing powder containing lead oxide, boric oxide, and silicon dioxide to produce a free-flowing mixture, the proportions of graphite to glazing powder within the mixture being in excess of 1 to 2, applying a thin layer of said mixture over aselected portion of the surface of the luminescent layer between the electrodes, and sintering said layer on saidluminescent layer at a temperature of approximately500 C. to produce a firmly adherent con ductive stripon said luminescent layer.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF PROVIDING A STARTING STRIP FOR AN ELECTRIC GASEOUS DISCHARGE TUBE INCLUDING A GLASS ENVELOPE, A PAIR OF SPACED ELECTRODES IN THE ENVELOPE, AND A LUMINESCENT LAYER ON THE INTERNAL SURFACE OF THE ENVELOPE WHICH COMPRISES FORMING A SUSPENSION OF FINELY-GROUND GRAPHITE IN AN ORGANIC SOLVENT CONTAINING AN ORGANIC HEAT- 